In U.S. Pat. Nos. 5,526,609 and 5,765,313 to Lee et al. there is disclosed method and apparatus for effecting a real time structural parameter modification (RSPM) for protecting a structure such as a high rise building or the like from the harmful effects of an earthquake or high winds. In this type of system a series of fluid springs are coupled to the structure at key locations by hydraulic switches. When a switch is activated by high external forces the spring engages an element of the structure that is being deflected so as to stiffen the element. Energy is thus stored in the spring and is later released from the system when the spring is disengaged from the structural element. In the RSPM system the element is released when its deflection reaches a zero velocity. The element is again engaged as it deflects in the opposite direction.
A fluid spring unit for protecting a substructural element from the harmful effects of a cyclic event is disclosed in U.S. Pat. No. 6,324,794. The unit is contained within a main structure and includes a housing containing a fluid filled cavity that is separated into two chambers by a piston that is slidably mounted within the cavity. The housing is attached to one of the structures and the piston is connected to the substructure by a piston rod. The fluid spring is placed in fluid flow communication with a pressurized accumulator tank by a control circuit that is under the control of a microprocessor. A sensor monitors the pressure in the spring chamber. At the outset of a cyclic event the fluid in the chamber is permitted to store energy until the chamber pressure reaches a given level whereupon a control valve is opened allowing the high pressure fluid in the spring chamber to pass into the accumulator tank where its energy is dissipated rapidly in the form of heat.
Although these prior art devices work well in isolating structures from the harmful effects of a cyclic event, their response times are relatively slow and the amount of energy that can be absorbed is limited by the amount of space that is available for mounting the system components.